The present invention is directed to a high intensity discharge (HID) lamp, and more particularly a ceramic metal halide (HCI) lamp, that offers better control of the temperature of the frit seal.
Ceramic lamp envelopes (also referred to as arc tubes or discharge vessels), such as those composed of polycrystalline alumina, are designed to operate at specific wattages that result in unique temperature profiles at varying locations along the ceramic envelope. The arc chamber of an HCI lamp has a specialized salt fill, gas composition, and pressure and is designed to operate at a specified wattage to result in specific lumens per watt, color temperature and color rendering index (CRI). These attributes depend on the operating temperature of the arc chamber. If the arc chamber is too cool, some of the salts can condense, which affects the luminous flux, color temperature and CRI. Alternatively, arc chambers that are too hot can degrade the frit seal between the electrode system and the ceramic envelope, particularly when the frit is in contact with corrosive metal halide fills. This can result in degradation or failure of the lamp.
In HCI lamps, the frit seal is at an end of a longitudinally extended hollow ceramic capillary through which an electrode protrudes into the arc chamber. The frit seal seals the space inside the capillary between a metal electrode feedthrough and the interior ceramic wall of the capillary. A typical frit material used to seal ceramic envelopes is Dy2O3—SiO2—Al2O3.
One approach to solving the problem of the degrading frit seal is to replace the frit seal with another sealing structure, so that the lamp is “frit-less.” U.S. Pat. No. 5,861,714 (Wei et al.) discloses a HID lamp with a seal for the electrode feedthrough that does not include a frit. With reference to FIG. 1, the envelope includes a translucent ceramic tube 41 having first and second ends 42, non-conducting cermet end plugs 46 closing each of the first and second ends, and metal electrode feedthroughs 48 passing through the cermet end plugs. The cermet end plugs 46 have a multipart structure, where each plug has at least four axially aligned parts 46a-e with different coefficients of thermal expansion. In the embodiment shown in FIG. 1, the temperature load is reduced by moving the plugs a distance from the hot arc chamber of the lamp at the ends of ceramic capillaries 45. Separating the end plug from the arc chamber with the capillary reduces the temperature of the end plug by about 200° C. A gas tight connection between the capillary and the plug may be achieved with a bushing 47 surrounding the contact zone between the capillary 45 and the plug 46. The bushing 47 sealingly holds (clamps) the plug 46 to the end of the capillary 45. Wei et al. indicate that a frit may be applied to the outer surface of plug 46, but only to further amend the gas-tightness of the seal not as the seal itself.
Nevertheless, it is still desirable in many lamps to use a frit seal and the present invention offers better control of the temperature of the frit seal.